Lower olefins, such as propylene and isobutylene, can be produced by dehydrogenating lower alkanes. Various methods include industrially practiced dehydrogenation reactions using platinum catalysts, noble metal promoted zinc aluminate spinel catalysts, or chrome-alumina catalysts. However, these catalytic processes may suffer from two drawbacks. First, it is difficult to obtain high olefin yields due to equilibrium limitations of the dehydrogenation reaction. Second, the high temperatures typically required for these processes tend to degrade the catalyst.
One type of catalyst commonly used for dehydrogenating lower alkanes is an alumina supported chromia catalyst. Although this catalyst has a relatively high dehydrogenation activity, it may suffer from rapid coke formation during the dehydrogenation reaction. Consequently, frequent high temperature regeneration cycles are required. Due to the need for frequent regeneration, catalysts having a high degree of hydrothermal stability are desired in order to prevent frequent and costly catalyst replacement.
The rapid coke formation and frequent regeneration also necessitate the employment of cyclical processes, such as the Houdry process, when using chromia-alumina as a dehydrogenation catalyst. Cyclical processes make use of parallel reactors that contain a shallow bed of chromia-alumina catalyst. The feed is preheated through a fired heater before passing over the catalyst in the reactors. The hot product is cooled, compressed and sent to the product fractionation and recovery station. To facilitate continuous operation, the reactors are operated in a timed cycle. Each complete cycle typically consists of dehydrogenation, regeneration, reduction, and purge segments. A further requirement for continuous operation is the use of a parallel set of reactors, such as 3 to seven reactors. In an effort to circumvent equilibrium limitations, the reactors are operated at sub-atmospheric pressures during the dehydrogenation cycle (2 to 14 psia). Regeneration is performed with pre-heated air through a direct fire burner or with the exhaust of a gas turbine. Regeneration temperatures range from 550 degrees Celsius to 750 degrees Celsius.
Because of such severe operating conditions, dehydrogenation catalyst life is typically one to less than two years. Catalyst replacement is performed when conversion and selectivity fall below minimum levels required for the economic operation of the unit. For example, a dehydrogenation catalyst may have an initial conversion and selectivity values of 50-60% and 88-90%, respectively, while end-of-life conversion and selectivity values are typically 40-45% and 75-85%, respectively.